The Pennsylvania Muscle Institute is an interdisciplinary center for research on muscle related to heart disease, and supported by HL 15835. This proposal is for the following specific studies on the mechanics and energetics of smooth and cardiac muscle contraction: 1) To further investigate the rigor state in vertebrate smooth muscle with emphasis on the similarity between rigor and resting conditions. 2) To determine the energetics of crossbridges proposed to be attached in resting smooth muscle when normal quantities of calcium are present in the external medium. 3) Quantitation of the rates of decay of tension after an imposed length change in resting and rigor smooth muscle to determine whether or not the same underlying process accounts for the fall in tension. 4) The ATP breakdown associated with force development and maintenance at different lengths with respect to l o will be measured in mammalian smooth muscle under defined mechanical and chemical conditions. 5) Experiments will be done to determine the efficiency with which mammalian smooth muscle converts the free energy of ATP hydrolysis into work. The velocity dependence, if any, of the efficiency will also be determined. 6) Comparison of the ATP usage and force maintenance during stretching under resting and activated conditions. 7) Study of the rate of utilization of chemical energy as a function of myocardial work in rabbit left ventricular papillary muscle from normal hearts and hearts subjected to pressure or volume overload. 8) To continue to provide a service to other members of the Pennsylvania Muscle Institute in the measurement of nucleotides and other metabolities in muscle samples.